Got the answer from Ron de Bruin's site
"Outlook Express
If you configure Outlook Express as the default mail handler (or simple MAPI client) on the
General tab, Outlook Express processes requests by using Simple MAPI calls.
Some viruses can exploit this functionality and spread by sending copies of e-mail messages
that contain the virus to your contacts.
By default, Outlook Express 6 prevents e-mail messages from being sent programmatically from
Outlook Express without your knowledge by displaying a dialog that enables you to send or not
send the message.
If you regularly use an application that uses Simple MAPI calls to send e-mail as yourself,
you may want to disable this protection as follows:
1 : Start Outlook Express, and then on the Tools menu, click Options.
2 : Click the Security tab, and then click to remove the check mark from the warn me when other
applications try to send mail as me check box.
3 : Click OK to close the Options dialog box.